It is known that the incorporation of wood meal, cellulose fibers and other cellulose based fillers and reinforcing substances in thermoplastic polymers significantly improves a number of mechanical properties in products made of such mixtures. For example, the modulus of elasticity of such mixed products can be increased up to triple the modulus of elasticity of the starting thermoplastic polymer material, depending on the amount of wood meal involved and on the particular thermoplastic polymer material employed. Such a value can be reached, for example, with the use of an impact resistant polystyrene containing 60% wood meal.
It is also known that the bending strength of such mixed products can be improved to over twice the value of the unfilled polymer. A further known advantage of products made from a mixture of a thermoplastic polymer material and wood meal is that the coefficient of thermal expansion of these products are reduced by about 50% and, moreover, as extruded profiles, they exhibit less tendency to shrink at higher temperatures.
All these improvements are of a mechanical nature and impart broad applicability to such products. The improved properties attained are attributable to the particular mixture from which the products are made and are generally not realized to such an advantageous degree from comparable products made completely of the unfilled thermoplastic polymer.
The publication "Kunststoffe" ["Plastics"], in volume 65 (1975), on pages 69 et seq. and particularly on page 71, discloses the quantities of wood meal that are considered suitable for the extrusion of various thermoplastics. For example, an addition of 30% wood meal is proposed for the extrusion of high pressure polyethylene at a mass temperature of 180.degree. C. For polypropylene, at the same mass temperature, the proportion increases to 70% wood meal while for polystyrene, at a mass temperature of 200.degree. C., the use of up to 60% wood meal is suggested. Further, according to this reference, polyvinyl chloride, at a mass temperature of about 160.degree. C., is able to handle only a 20% portion of wood meal.
While the addition of wood meal to thermoplastic polymers in the above-mentioned quantity ranges produces a favorable increase in mechanical properties of extruded profiles or injection molded pieces over pieces formed from non-filled thermoplastics, the filling of such polymers with the respective amounts of wood meal considerably reduces their electrical insulating values. As a result, profiles or molded pieces filled with wood meal or other cellulosic material have not been previously used as insulating material in the electrical industry.
The publication "Modern Plastics International", Feburary, 1974, at pages 22 to 24 discloses PVC-wood flour mixtures containing 33% wood flour. A number of uses are disclosed for such mixtures, but use as an insulating material in the electrical industry is not disclosed.